Modem electronic products including computers, telecommunication equipment, automobile electronics, and consumer electronics require circuit interconnection. The off-chip circuit interconnection and packaging is typically made with solder joints, either by wave soldering or surface mounting technique, as is well known in the art. An increase in circuit interconnection density is desirable for lowering the device cost as well as for miniaturizing the electronic products, which has been the general trend in recent years.
In high or ultra-high density circuit connections, the size of the interconnection contact pads and hence the size, especially the height of solder joints on them, is drastically reduced. As a result, the shear strain which is related to the ratio of the lateral change in substrate dimension (e.g., caused by thermal expansion mismatch) over the solder joint height and accompanying stress on solder are significantly increased. In the widely-used eutectic Pb-Sn solder, it is well known that the solder joint failure by fatigue is caused by the combination of cyclic temperature change and extensive plastic shear strain which in turn causes microstructural coarsening and crack initiation. If the solder joints had high-aspect ratios and were elastically compliant, their elastic bending could have accommodated some of the strain and minimized the amount of plastic strain on solder. However, most present-day solder joints are relatively short (e.g. spherical segment geometry) and, hence, deform plastically with little elastic compliance. In view of these considerations, there is a need for improved solder joint geometry to reduce the shear strain and to enhance the joint reliability during the service of electronic products.